1. Field of the Invention
This invention relates to a sensor assembly and method for determining the condition of a structure, especially for confirming if a measured fingerprint is on a live finger, by measuring characteristics of close to the structure surface.
2. Introduction
Capacitance or impedance based fingerprint sensors are some of the most promising approaches to offer a low-cost, miniaturized device for biometric identification. Such sensors are therefore possible candidates for integration in mobile phones etc.
To enhance the trust of fingerprint sensors, it is of great importance that any attempt to fool the system by using fake fingers may be detected and rejected. A fake finger will typically consist of a slab of material with electrical properties resembling that of a finger, and with a fingerprint carved or molded into its surface. In a more extreme case one may also imagine that a dead, cut-off finger may be used.
For live finger detection systems, it is important both that the probability of accepting a false finger (false acceptance ratio—FAR) and the probability of rejecting a real finger (false rejection ratio—FRR) are extremely low. This makes it important to develop a method for identifying very characteristic and unique properties of a living finger, properties that can not easily be replicated by either synthetic materials or by other biological substance than living tissue, and which are typical of the vast majority of fingers in the population.
For low cost capacitive based fingerprint sensors some kind of impedance measurement of the finger properties will be ideal, as it can most often be integrated directly on the device by using existing measurement structures or adding a number of extra electrodes.